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The construction and performance of a single stage nuclear demagnetization apparatus utilizing 0.267 mole of praseodymium nickel material is described. An optimization procedure to minimize entropy production during the demagnetization of the hyperfine enhanced material has been developed which enabled the cryostat to cool a liquid-3He sample to a temperature of 350 μK. The effects of measured heat leaks and inferred eddy-current heating are accounted for satisfactorily in a model of this procedure.

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By means of the first observations of the quantum Hall effect in a type II–VI semiconducting compound, Hg1−xCdxTe, we have studied the two-dimensional electron gas formed by the inversion layer of a MISFET (metal–insulator–semiconductor field effect transistor) device. Extensive details regarding the fabrication and use of the MISFET are described. The data also indicate an abrupt onset of the quantum Hall effect which, when interpreted with a percolation threshold theory, is used to further investigate conditions affecting charge transport mechanisms in the two-dimensional electron gas at a HgCdTe interface.

A high-precision NMR technique was used to measure the magnetic susceptibility of bcc solid ^He at several molar volumes over a temperature range well above the ordering temperature. The results resolve a long-standing inconsistency between high-magneticfield pressure measurements and earlier magnetic susceptibility measurements, give new values of the Curie-Weiss temperature 6 nearly a factor of 2 smaller than previous values, and suggest that 6 follows a power-law volume dependence.PACS numbers: 67.80.Jd, 67.80.Gb, 75.30.Cr The magnetic behavior of solid ^He is believed to be a consequence of quantum exchange of whole atoms with a nuclear magnetic moment. This situation is unusual and interesting compared to other solids where magnetic effects rely on quantum exchange of electrons. Despite considerable theoretical and experimental efforts to understand magnetism in solid ^He, a surprising number of unresolved problems still challenge our basic knowledge of this quantum solid. This Letter describes an experiment where new and unexpected results have been found using high-precision NMR measurements to study the magnetic susceptibility of solid ^He. The results resolve an inconsistency between previous high-magnetic-field pressure measurements^'^ and magnetic susceptibility measurements,^*^ show that the Curie-Weiss temperature 6 is about a factor of 2 smaller than the values found in earlier susceptibility work, and provide evidence in support of a power-law volume dependence for B. Samples were investigated at various molar volumes (21 to 24 mL/mole). The measurements were done in the zero-field limit with a low magnetostatic field (17.1 mT), covering a temperature range of « 16 to 514 mK, A model-independent spin-exchange Hamiltonian is useful for developing a high-temperature series expansion of the magnetic susceptibility x. In the limit of zero magnetic field, the inverse susceptibility can be written as X'' = C-HT-0+J5/T),where higher-order terms in temperature T have been excluded.^ The Curie constant C and 9 are familiar quantities; B, on the other hand, is a coefficient related to ^ by B = ^^ -0^2/8> where a2 is a second-order coefficient occurring in the high-T expansion of the Helmholtz free energy.Accurate values of 0 have not been available from past susceptibility experiments.^'^ One group of experiments^ were done at high temper-atures (« 10 to 500 mK) where the precision was insufficient to determine accurate values of 0 because of very small deviations from Curie-law behavior. Another group of experiments^ were done at much lower temperatures (« 1 to 15 mK) near the ordering temperature where significant higher-order deviations from Curie-Weiss behavior developed. However, because of correlations among higher-order terms in Eq. (1), values of B could easily be misinterpreted from the data since high-temperature behavior was not studied.One can also compare susceptibility measurements with high-magnetic-field pressure measurements P{T;H),^'^ The latter are sensitive to the volume derivatives of...